Electronic parts which include a resistive or dielectric substrate coated with a thin layer of conductive metal find important use in industry. The conductive metal layer is applied to the substrate by spraying, brushing, dipping, rolling or screen printing a coating composition of finely divided, metal particles suspended in a liquid vehicle. Spraying is a preferred method of applying the conductive metal layer because it is fast and it permits laying down uniform, thin layers on very intricately shaped parts. Subsequently, the coating composition is dried at ambient or slightly higher temperature, then the part is fired at very high temperature to permanently bond the metal to the substrate.
Traditionally, the solvent in the liquid vehicle for silver coating compositions has been primarily a volatile organic compound, (VOC). A VOC-based vehicle has several drawbacks. It may be toxic, flammable, and possibly explosive. The VOC raw material is costly to purchase and the waste is expensive to discard. Generally, VOC solvent emitted from coating operations is an environmental air pollutant. Expensive equipment and procedures are required to capture and contain VOC solvent emissions in order to reduce air pollution and enable proper waste disposal.
While it is desirable to have a sprayable, silver coating composition based on an aqueous vehicle, heretofore it has been unknown how to formulate such a composition. Lubricant on the silver is one obstacle to achieving aqueous based coating compositions. Silver flake is usually made by milling lubricated silver powder. The lubricant is very important for assuring that the metal particles form into flakes as a result of the milling. It has been found that residual lubricant on the flake surface can make the flake hydrophobic and difficult to disperse in water. Poorly dispersed silver flake will not spray uniformly, and produces an irregular coating on the substrate. Furthermore, it is common in the industry to use very small hole spray nozzles. Poorly dispersed silver coating composition can frequently clog small hole nozzles. It is also desirable to have a very high concentration of silver particles in the composition in order to lay down at high speed a specified coating thickness in the least number of spraying applications, and preferably only one application. However, if silver particle concentration is too high, the coating composition viscosity rises excessively and adversely affects spraying performance. High silver concentration also increases the tendency of the silver to settle and irreversibly agglomerate. Prior to this invention, a low viscosity, highly silver-loaded, sprayable coating composition based on an aqueous vehicle was unavailable.
U.S. Pat. No. 5,286,415 discloses an aqueous conductive polymer thick film-forming composition useful for screening into electrical circuits and for formulation into human electrodes. The disclosed composition comprises five essential ingredients, namely, (a) water soluble thermoplastic polymer; (b) polymer dispersion in water; (c) glycol drying retarder; (d) electrically conductive particles; and (e) water. The dispersed polymer component comprises from 10 to 70 wt % of the composition. The patent discloses that the composition is preferably 8,000-25,000 cps (8-25 Pa.s) which is the ideal for screen printing. Although the composition may be sprayed, the number and concentrations of the essential ingredients suggests that viscosity of the patented composition will generally be too high for spray application. It is desirable to have an aqueous silver composition which has fewer, minimum essential ingredients, and which is therefore simpler, faster and less expensive to produce. A composition with viscosity less than 8,000 cps is desirable to allow spray coating a conductive layer of silver on a substrate in the fewest passes, preferably, in a single pass.
Accordingly, it is a main objective of this invention to provide for the safe, environmentally benign production of silver-coated, ceramic parts. It is a feature of this invention that a silver coating composition contains an aqueous liquid vehicle which is substantially free of volatile organic compounds. The present invention advantageously allows a silver coating composition to be coated on ceramic parts without the need to recover and safely dispose of large quantities of volatile organic compounds.
It is another objective of the present invention to provide a silver coating composition that can be applied using conventional coating equipment and without changing existing manufacturing procedures significantly. It is an advantage of this invention that well-dispersed, aqueous coating compositions based on lubricated silver flake can be formulated. It is a feature of this invention that the coating compositions have an optimum combination of low viscosity and very high loading of silver flake. Therefore it is another advantage of the present invention that silver coating compositions can be applied in only a single, high speed spray application using existing, small hole, spray nozzles to provide a very uniform, firmly attached, metal coating.
It is still another objective of the present invention to provide a stable coating composition in which the silver flake resists separation from the liquid vehicle on standing in storage. The present invention includes a combination of ingredients which promotes and maintains uniform dispersion of the silver in the aqueous-based vehicle. It is an additional feature of this invention that any solids which settle can be readily redispersed in the vehicle with minimum agitation.
Accordingly, there is presently provided a coating composition consisting essentially of:
(a) about 30 to about 80 wt % silver flake in the form of lamellar particles having a long dimension of at most about 15 .mu.m coated with a lubricant comprising at least one straight-chain carboxylic acid or salt of said acid, said lubricant having from 6 to 18 carbon atoms; PA1 (b) about 1.5 to about 4.5 wt % substantially completely water soluble polymer binder selected from the group consisting of acrylic polymer and acrylic copolymer; PA1 (c) a solvent effective amount of water;
wherein the percentages are based on total weight of the coating composition
When it is desired to permanently attach the silver layer to a suitably temperature resistant substrate, such as ceramic, the composition can contain an optional, sintering adhesive component and the coated part can be fired at a temperature from about 600.degree. C. to about 950.degree. C. for a duration sufficient to activate the sintering adhesive component. Subsequently, the part is cooled to solidify the sintering adhesive, thereby bonding the silver coating to the substrate.